1. Field of the Invention
The invention relates to an improved class of protective coatings for use on superalloy articles, such as gas turbine rotating blades and stationary vanes.
Wide use of single crystal (SX) and directionally solidified (DS) hot-stage components has allowed increased turbine inlet temperature and therefore turbine efficiency. The improvement in high-temperature strength of these new superalloys involved an increased susceptibility of the alloy to sulfidation and oxidation. To restore environmental resistance to engine parts made from DS and SX alloys requires a new generation of high-temperature resistant coatings. Historically, aluminide or MCrAlY coatings (where M represents a transition element such as Ni, Co, Fe or mixtures thereof) have been applied by engine manufacturers to extend the useful life of hot section components.
Due to their limited thickness (typically around 50 .about..mu.m) aluminide coatings do not offer sufficient oxidation and corrosion protection for the long exposure times in stationary gas turbines (20000-50000 hours). Present MCrAlY coatings, in particular when the Al reservoir phase consists of .beta. (NiAl) phase demonstrate much greater environmental resistance compared to aluminide coatings. However, since a coated turbine blade undergoes complicated stress states during engine operation (especially during start up and shut down) advanced high temperature coatings must not only provide environmental protection but must also have specifically tailored physical and mechanical properties to provide high thermo-mechanical fatigue resistance. In summary, high-temperature resistant coatings must meet the following requirements:
high oxidation resistance PA1 slowly growing oxide scale and good oxide scale adherence PA1 hot corrosion resistance, superior to SX/DS superalloys PA1 low interdiffusion of Al and Cr into the substrate to prevent the precipitation of brittle needle-like phases under the coating PA1 high thermo-mechanical fatigue resistance
U.S. Pat. Nos. 5,273,712 and 5,154,885 disclose coatings with significant additions of Re which simultaneously improves creep and oxidation resistance at high temperatures. However, the combination of Re with high Cr levels, typical for traditional coatings, results in an undesirable phase structure of the coating and interdiffusion layer. At intermediate temperatures (below 950-900.degree. C.), .alpha.-Cr phase is more stable in the coating than the .gamma.-matrix. This results in low toughness and low ductility. In addition, a significant excess of Cr in the coating compared to the substrate results in diffusion of Cr to the base alloy, which enhances precipitation of needle-like Cr-, W- and Re-rich phases.
U.S. Pat. No. 4,447,503 discloses a superalloy coating composition with high temperature oxidation resistance. The coatings consist essentially of, by weight, 5-50% Cr, 3-30% Al, 0.01-15% Ta, up to 10% Mn, up to 5% W, up to 12% Si, up to 10% Hf, up to 5% reactive metal from the group consisting of La, Y, and other rare earth (RE) elements, up to 5% of RE and/or refractory metal oxide particles, and the balance selected from the group consisting of Ni, Co and Fe, and combinations thereof. Additions of up to 5% Ti and up to 15% noble metals are also contemplated. However, the coatings are only intended for applications where the need for improved high temperature oxidation is paramount and where the coating ductility is relatively unimportant.